The human platelet Fcgamma receptor plays an important role in several hematologic disorders characterized by thrombocytopenia, thrombosis or vasculitis. Over the past 15 years, knowledge of the platelet Fcgamma receptor has grown. Aspects of its genetics, regulated expression and structure/function relationships, as well as its function in rive in immune pathology, have been studied by our group and others. The long term goai of our project is to more fully decipher the molecular mechanisms underlying piatelet Fcgamma, receptor biology in order to provide safer and more rational therapy for the immune platelet disorders, in addition, studies of the genetics, regulated expression, and signaling pathways of the platelet Fcgamma receptor remain synergistic with the other projects of our Program Project. The Specific Aims of this proposal are: 1. To examine the role of platelet FcgammaRIIA vs macrophage FcgammaRIIA in immune clearance in vivo. Our hypothesis is that the expression of FcgammaRllA ptays a major rote in regulating the fate of immune complexes in vivo, and that abrogation of ptatelet Fcgamma receptors, despite continued macrophage expression, will lead to aberrant clearance of immune complexes. Platetet-specific FcgammaRIIA transgenic mice as well as macrophage-specific FcgammaRIIA transgenic mice will be generated by use of DNA constructs with celt-specific promoter sequences appended to the coding region of the FcgammaRIIA gene. 2. To examine the regulation of expression of the platelet Fcgamma receptor. The level of platetet FcgammaRIIA correlates with its function in health and disease. Our hypothesis is that genetic variation in FcgammaRllA gene regulatory sequences, such as the promoter single nucleotide polymorphisms (SNPs) we recently identified, determine differences in the level of FcgammaRIlA expression. The mechanism of the SNP effects on FcgammaRllA expression will be studied in vitro with transfected reporter genes driven by genetic isoforms of the FcgammaRllA gene in megakaryocytic ceils, and in vivo in new transgenic mouse lines created with FcgammaRlIA variants. 3. To examine the role of Cbl in immune clearance, endocytosis and platelet activation. We have established the importance of Cbl in FcgammaRIIA mediated endocytosis in vitro and have crossbred Cbl knockout mice to our FcgammaRIIA transgenic mice. Our hypothesis is that Cbl is important for both FcgammaRIIA endocytosis in rive and FcgammaRIIA subcellutar localization. Continued progress in platelet Fcgamma, receptor biology should permit better diagnosis and therapy for patients with immune ptatelet disorders.